Inverter power supply systems are widely applied to various scenarios that require reliable power supply. For example, in the communications field, an inverter, an uninterruptible power supply (UPS), or the like is generally used to supply power to a key alternating current load, and a typical inverter power supply system is shown in FIG. 1. An inverter power supply generally has three power ports: a direct current (DC) battery input port, an alternating-current input port, and an alternating-current output port, and the ports correspond to three power converters: a DC/DC boost converter that boosts a low battery voltage to a high direct-current bus voltage, an alternating current (AC)/DC power factor correction (PFC) converter that converts a mains AC to a direct-current high-voltage bus and that completes input power factor correction, and a DC/AC inverter that converts a high-voltage direct current to an alternating current AC for output. A filter capacitor having a relatively large capacity is connected in parallel to a direct-current bus.
Input of a three-phase UPS is generally characterized by a three-phase four-wire mechanism plus a ground wire, that is, an A-phase wire, a B-phase wire, a C-phase wire, a zero wire, and a ground wire. When the three-phase UPS cannot receive a signal on a zero wire of an alternating-current power network, that is, the zero wire is lost, the three-phase UPS needs to be capable of detecting loss of the zero wire, and switch to a new control method, to ensure that the three-phase UPS can normally work with full load. After the zero wire is lost, if no measure is taken, a Neutral-ground voltage, that is, a difference between voltages of the ground wire and the zero wire, is very large, this voltage directly acts on a load and may damage the load. Moreover, after the zero wire is lost, because a middle point potential of positive and negative buses of the three-phase UPS are uncertain, the buses are probably unbalanced, that is, a difference between absolute values of voltages of the positive and negative buses is relatively large, and as a result, a voltage of one bus is excessively low and power-off is caused, which seriously affects reliability and a usage scope of the three-phase UPS.
Currently, when a PFC rectifier in a three-phase UPS has a two-level topology, after a zero wire is lost, a 0-axis reference current i0* in a dq0 coordinate system is generally set to 0, then, a 0-axis current i0 (i0=ia+ib+ic)/3) is sampled, where ia is an A-phase current of a three-phase alternating current received by the three-phase UPS, ib is a B-phase current of the three-phase alternating current received by the three-phase UPS, and ic is a C-phase current of the three-phase alternating current received by the three-phase UPS, to obtain a deviation Δi0=i0−i0* between 0-axis currents. Proportional-integral (PI) adjustment, is performed on the obtained deviation Δi0 between 0-axis currents, to generate a 0-axis direct-current modulated wave, where an amplitude of the 0-axis direct-current modulated wave is proportional to a Neutral-ground voltage. Therefore, the Neutral-ground voltage can be controlled within a particular range by limiting the amplitude of the 0-axis direct-current modulated wave.
However, if a PFC rectifier in a three-phase UPS has a three-level topology, after a zero wire is lost, if the current control method is used, a 0-axis direct-current modulated wave is equivalent to a direct-current voltage applied to a neutral point of a transverse bridge and a longitudinal bridge of the rectifier, and as a result, a current that flows through the transverse bridge in an alternating-current positive half cycle and a current that flows through the transverse bridge in an alternating-current negative half cycle are unequal; therefore, absolute values of voltages on positive and negative buses are unequal, and finally, a voltage of one bus is excessively low and power-off is caused.
In conclusion, after a zero wire is lost, when the current control method is applied to a three-phase UPS using a three-level topology, a 0-axis direct-current modulated wave results in that absolute values of voltages on positive and negative buses are unequal, and as a result, a voltage of a bus may be excessively low and power-off may be caused.